The effect of strategy on pseudoneglect for luminance judgements

When judging the relative magnitude of the left and right sides of a stimulus, normal participants overestimate the leftward features (pseudoneglect). Although pseudoneglect and clinical neglect operate in opposite directions, the two phenomena may have a common cognitive and neural basis. For neglect, two strategies may be employed when inspecting horizontally aligned stimuli: (1) A global strategy where the stimuli are treated as a gestalt and asymmetries are detected or, (2) a comparison strategy where the qualities on the left and right sides of the stimuli are explicitly compared. To investigate the effect of these strategies on pseudoneglect, normal dextrals (n = 25 and 17) made two-alternative, forced-choice luminance discriminations between two mirror-reversed luminance gradients (greyscales). In an unseparated form, the stimuli are amenable to a global strategy. A comparison strategy was imposed by separating the stimuli into halves (Experiment 1) or quarters (Experiment 2). Despite the fact that the stimuli were equiluminant, participants predominantly chose the stimulus that was dark on the left as being darker overall in the unseparated condition. Response times were also faster for leftward responses. When the stimuli were separated into halves or quarters, the leftward bias was reduced, but not eliminated. The results demonstrate that both strategies contribute to pseudoneglect--though the global strategy may produce stronger pseudoneglect.